Add `@triagebot squash` command

[jyn514]

A common challenge new contributors have is squashing commits into one:

• This requires having a local clone of the repo (especially for doc PRs, most people just use the web interface)
• Knowing how to use git rebase, including that you should use -i to edit individual commits
• Knowing that git push -f is ok (this last step trips a lot of people up - they'll rebase and then 'merge branch into master' which defeats the whole point)

Even for existing contributors squashing is annoying because it messes up your build cache, since cargo works on mtimes and not hashes.

It would be great to have a squash command that automatically squashes all commits in the current PR into one. The way to do this locally is

git checkout https://github.com/rust-lang/rust/pull/NNNNN && git rebase -i origin/master && git push -f git@github.com:CONTRIBUTOR/rust.git HEAD:BRANCH

NNNNN is the pr number, CONTRIBUTOR is the author, BRANCH is the branch name; all of these are easily available from the PR itself.

[jyn514]

This first made me annoyed enough to open an issue in rust-lang/rust#76289, but it's happened a lot before that too.

[camelid]

@rustbot modify labels: enhancement

[camelid]

git checkout https://github.com/rust-lang/rust/pull/NNNNN && git rebase -i origin/master && git push -f git@github.com:CONTRIBUTOR/rust.git HEAD:BRANCH

@jyn514 Hmm, are you sure this command is correct? Running (for example)

$ git checkout https://github.com/rust-lang/rust/pull/77508
error: pathspec 'https://github.com/rust-lang/rust/pull/77508' did not match any file(s) known to git

fails for me.

Also:

• We probably need to git fetch first
• We should use git rebase -i $(git merge-base master HEAD) so we don't introduce merge conflicts
• We should use --force-with-lease so we don't overwrite new commits

[jyn514]

Sure, all of those seem fine. I never actually tested these commands.

[Mark-Simulacrum]

You need to install hub (GitHub integration for git) if you want the command to work without fetching.

[camelid]

Or you can use the newly-stable gh tool!

[kellda]

What about https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/about-merge-methods-on-github#squashing-your-merge-commits ? (GitHub allows to squash commits while merging)

[pietroalbini]

We need to merge PRs through @bors, we can't use the merge button on rust-lang/rust.

[kellda]

cc servo/homu#70

[kellda]

I implemented this in @bors (rust-lang/homu#122) because it already has a local git clone and is performing the merge. Is this still wanted in triagebot ?

[Mark-Simulacrum]

I think the homu support is great but I am a bit wary of r+'ing PRs that are then going to get squashed by homu at a later point. But generally I think support in homu is probably enough, so I'm going to close this. We can reopen if there's problems with the homu support at some point.